how do I iterate through internal properties in c#

You need to specify that you don’t just need the public properties, using the overload accepting BindingFlags:

foreach (PropertyInfo property in typeof(TestClass)
             .GetProperties(BindingFlags.Instance | 
                            BindingFlags.NonPublic |
                            BindingFlags.Public))
{
    //do something
}

Add BindingFlags.Static if you want to include static properties.

The parameterless overload only returns public properties.